1. Field of the Invention
The present invention relates to acrylic resin which is obtained through suspension polymerization such as Poly(Methyl MethAcrylate) which is referred to as PMMA hereinafter.
2. Description of the Prior Art
The PMMA is a plastic material which is known as an organic glass. The PMMA has been recently used for a transparent substrate of an optical recording disk. This is because the PMMA is readily formed into a desired shape such as a disk by casting and the resultant has a sufficient strength although it is somewhat inferior to an inorganic glass in optical properties.
The PMMA is supplied to the market in the form of monomer for casting or of pellets for injection molding. The substrate obtained through the casting has a crosslinking structure because of the conversion of monomer to polymer. The substrate has therefore a good hardness at its surface and a good thermal property. The polymerization reaction during the casting is exothermic and causes some shrinkage so that the resultant substrate necessarily has strains. Thus, the polymerization reaction should be retarded to avoid the strains in the substrate, and therefore the productivity becomes low, so that the cost for manufacture of the substrate has been relatively high.
Nowadays the transparent substrate is manufactured via injection molding from pellets of PMMA which are obtained through suspension polymerization which is used mainly for the production of PMMA.
Suspension polymerization is a method in which polymerization is conducted in a liquid (water is mainly used) while monomers are suspended therein together with an initiator and a stabilizer. The polymerization is carried out in beads of suspended monomers in the liquid which absorbs heat of reaction. The beads of polymer are precipitated after stirring. This method is widely used in the industry of plastic because of easiness of separation of the produced beads of polymer from the liquid.
A starting material of monomer such as methyl methacrylate, ethyl acrylate, methyl acrylate or the mixture thereof is used in suspension polymerization. The starting material is added to water in a vessel together with a predetermined amount of additives such as a catalyst, a suspending agent and a stabilizer. These reactants are stirred at a predetermined temperature. Meanwhile, the suspension is cooled down and acidified to remove the suspending agent. The beads of polymer are then filtered off, washed and dried in air.
A standard process for manufacturing pellets of PMMA through suspension polymerization is schematically illustrated in FIG. 1.
The starting material of monomer, for example, methyl methacrylate, ethyl acrylate, methyl acrylate, etc. is added together with a catalyst, an initiator and a suspending agent into water in a reaction vessel 1. A surface active agent is added to these reactants for uniformly suspending beads thereof. A stabilizer, e.g. sodium sulfate (Na.sub.2 SO.sub.4) or sodium phosphate is also added to these reactants for stabilizing this reaction system. These reactants are stirred while being initially heated up to a predetermined temperature. The polymerization advances to produce beads of polymer in the form of slurry. The slurry is transferred to a treating and washing vessel 2 to be treated and washed so as to remove disused substances such as the suspension agent. The resultant slurry is transferred to a dewatering machine 3 to remove water. The slurry is concurrently transferred to a dryer 4 to be dried. The resultant cake is transferred through an extruder 5 to a pelletizer 6 which transforms the beads of polymer into pellets of PMMA.
The obtained pellets of PMMA are formed through injection molding by means of a predetermined die into a transparent substrate of an optical recording disk.
When however a video signal bearing surface is mounted on the substrate made from the pellets of PMMA in order to manufacture an optical video disk, a deterioration of the reproduced image with the so-called "color flash" sometimes occurs during a play-back operation of the manufactured optical video disk.
Inventors of the present application investigated the cause of the "color flash" phenomenon. As a result, the inventors have found that a defect 14 on the video signal surface, i.e., a recess 14a caused by a minute bubble remaining thereon damages the reproduced image of the optical video disk, as shown in FIG. 2. The size of each pit on the optical recording disk is approximately 1.0 .mu.m and of the same order as the minute bubbles. When the recess 14a caused by the bubble exists close to the pit 13 between the transparent substrate 11 and the reflection layer 12, the pit 13 can not be accurately read out by the laser spot 15. In this way, the color flash phenomenon occurs in the reproduced image of the optical video disk.